1. Field of the Invention
The present invention relates to a reflecting type liquid crystal display device.
2. Description of the Prior Art
Recently, liquid crystal display devices have been applied to various fields such as notebook type personal computers, monitors, car navigation, scientific electronic calculators, small and medium size TVs, etc. Among these, investigations into reflecting type liquid crystal display devices are being carried out, in order to take advantage of their low energy consumption due to not requiring a backlight, as well as their thinness and light weight for applications in displays for portable devices.
In reflecting type liquid crystal display devices, apart from the TN mode display mode which uses 2 polarizing plates, there are three main types of display modes; GH (Guest Host) mode, high molecular dispersion mode, and single polarizing plate mode.
The GH (Guest Host) mode has the special characteristic of being able to obtain a bright display due to relatively low light absorption. However, there is the disadvantage that, as the two-color ratio of the material cannot be adequately obtained, the contrast is low. The high molecular dispersion mode has the special characteristic of having a wide angle of visibility due to achieving satisfactory light diffusion. However, it has the disadvantage of low reflectance and contrast, and is not yet ready for practical use. Although the single polarizing plate mode has light absorption due to the polarizing plates and is unable to obtain satisfactory brightness, high contrast can be obtained as it uses polarizing plates, and it is becoming the main reflecting type color liquid crystal display device in current use.
An example of the single polarizing plate mode, seen from the observer side, is composed of a polarizing plate, a glass substrate, a drive liquid crystal formed from spirally orientated chiral nematic liquid crystal, a glass substrate, a reflecting plate and reflecting electrode. Both polarizer and analyzer functions are obtained by the incidents light""s making two transits of the liquid crystal layer and the polarizing plate when incoming and outgoing via the reflecting layer. For this reason, compared to the reflecting type TN mode which uses 2 polarizing plates and which is used in conventional calculators and watches and the like, by reducing the number of polarizing plates by 1, it has the special characteristics of having brightness and in addition, by using polarizing plates it has the same control effect and satisfactorily high contrast.
In addition, the surface of the reflecting layer of this kind of single polarizing plate mode is formed from a bumpy reflecting plate which has the function of diffusing and reflecting light, or a completely regular reflecting flat metal reflecting surface. With the former bumpy reflecting surface it is possible to obtain a simultaneous light diffusion function and a reflecting function, however, as the polarized state of the entering light is altered at the reflecting surface by the bumps, there is the problem of reduced contrast with the single polarizing plate mode construction which requires maintenance of the polarized state at the reflecting surface.
On the other hand, in the case of a flat metal reflecting surface which obtains regular reflection, although a separate light diffusing light diffusion layer is required, there is no polarizing break down, and it has the special characteristic of achieving high contrast. The characteristics of reflected light with this kind of speculum reflecting surface, comprising a light diffusion layer, depend on the light diffusion layer. In the past for this light diffusion layer there were, for example, mat system anticlear processing of polarizing plates, and a diffraction grating film arrayed so that a refractive index medium having refractive index anisotropy distributes the refractive index two-dimensionally. However, these light diffusion effects have strong wavelength dispersion characteristics, and when the light source is in an environment relatively near to parallel, the reflected light has a yellowish coloring in the regular reflection direction of the light source, and conversely in the direction receding away from the regular reflection direction, there is the problem of bluish coloring. The reason for this is that the light diffusion effect becomes stronger the shorter the wavelength of the wavelength dispersion characteristics of the light diffusion effect of the light diffusion layer.
In this way, in a reflecting type liquid crystal display device formed from a construction of a speculum reflecting layer and a light diffusion layer in a single polarizing plate mode, as wavelength dispersion characteristics of the light diffusion effect of the light diffusion layer are present, there was the problem of coloring of the reflected light. This type of problem is not limited to the single polarizing plate mode, but is an inevitably occurring problem in reflecting type liquid crystal display devices that use a light diffusion layer.
In conventional reflecting type liquid crystal displays, the direction of the surface reflection in the panel display and the largest reflecting strength direction of the reflecting surface correspond, and in practice, the visibility from the brighter looking direction was extremely low.
One object of the present invention is to provide a simple construction reflecting type liquid crystal desplay device and to realize a low cost device with the characteristic of the largest reflecting strength direction of the reflecting surface coming in a viewing angle direction from which the surface reflection cannot be seen.
According to an aspect of the reflecting type liquid crystal display device of the invention, the device having at least two substrates, a liquid crystal layer sandwiched between these substrates, a reflecting surface which reflects light passing through this liquid crystal layer, and a light diffusion layer which at least diffuses the light reflected by that reflecting surface,
wherein the light diffusion layer is formed of 2 types of refractive index medium with differing refractive indices, and when the refractive index of the first refractive index medium is nA(400) at light wave length 400 nm, and nA(700) at light wave length 700 nm, and the refractive index of the second refractive index medium is nB(400) at light wave length 400 nm, and nB(700) at light wave length 700 nm, then,
(nA(400)/nA(700))xc3x970.9xe2x89xa6(nB(400)/nB(700))xe2x89xa6(nA(400)/nA(700))xc3x971.1xe2x80x83xe2x80x83(1)
By the above construction, the wavelength dispersion of the light diffusion effect is reduced.
Further, in the aforementioned, the light diffusion layer is constructed of a fine particle dispersion layer in which fine particles of the second refractive index medium are dispersed in the first refractive index medium, or of a diffraction grating layer in which the first refractive index medium and the second refractive index medium are alternately arranged having regularity in a surface direction.
The light diffusion effect of the light diffusion layer mainly depends on the light diffusion effect and on the light diffraction effect due to the refractive index difference of the 2 types of refractive index medium.
FIG. 2 shows the principle of a case using this kind of light refraction effect. As in FIG. 2a, when medium n1 of refractive index n1 and medium n2 of refractive index n2 are combined, refraction occurs according to the ratio of the 2 types of refractive index. When the degree of this refraction is represented as an angle of refraction, in accordance with Snell""s law, it becomes as in FIG. 2b. Here, xcex8o is the angle of incidence and xcex8p is the final outgoing angle. Here, when the wave length dispersion characteristic of the ratio n1/n2 of n1 and n2 is large, the aforementioned coloring problems occur because the degree of refraction differs according to the wavelength of the incident light. So that the wavelength diffusion characteristics of these 2 types of refractive index come within the ranges of Formula (1), 400 nm is the short and 700 nm is the long wavelength visible light range, and when nA(400)/nA(700), which is the refractive index ratio of the first refractive index medium near both ends of visible light, is the same as or approaching nB(400)/nB(700), which is the refractive index ratio of the second refractive index medium, as xcex8p becomes constant, wavelength dispersion due to refractive index difference is reduced. By selecting the materials of the refractive index media, the coloring of reflected light can be suppressed to a range where it is practically not a problem.
On the other hand, the light diffusion principle in the case of a diffraction grating system is shown in FIG. 3. In diffraction, when 2 types of refractive index medium, n1 and n2, are alternately arranged in a surface direction and light passing through this layer passes through each of the refractive index media, a phase difference occurs, and this phase difference causes interference of the light, thus creating diffraction and diffusing the light. First order, second order, third order, . . . diffracted light occurs, according to the order of interference of the diffracted light. The direction of travel of each of the orders (in the drawing xcex8m (m=1, 2, 3, . . . )) depends on the refractive index distribution pitch and the incident light wavelength. Here, Im is light intensity of m order, m is the order, xcex is the light wavelength, p is the grating pitch and d is the thickness.
Also, the light intensity of each order depends on the refractive index difference (|n1xe2x88x92n2|) of the refractive index media, the layer thickness of the refractive index media and the wavelength of the incident light. Consequently, the wavelength dispersion of the light scattering intensity depends on the refractive index difference of the refractive index media. Here, the aforesaid coloring problem occurs because the degree of refraction differs according to the wavelength of the incident light when the wavelength dispersion characteristic of (|n1xe2x88x92n2|) is large.
From these facts, by selecting the materials of the refractive index media so that the wavelength diffusion characteristics of these 2 types of refractive index come within the range of Formula (1), the coloring of reflected light can be suppressed to a range where it is practically not a problem.
Furthermore, the present invention is a reflecting type liquid crystal display device in which the reflecting surface is a speculum, and which is provided with a light diffusion layer on the light incidence side of that reflecting surface.
The surface of the said reflecting layer is a reflecting surface that is a speculum and makes metallic reflections, and if it is of a construction provided with a light diffusion layer having a function which diffuses incident light from the said reflecting surface, that is to say, it reflects light entering to the observation (observer) side, high contrast can be obtained since the polarized state of the incident light at the reflecting surface is reflected in a maintained form. Thus, better performance can be obtained with a combined light diffusion layer and reflecting layer than with a construction using a bumpy reflecting plate.
Furthermore, the present invention is a reflecting type liquid crystal display device wherein the reflecting layer is located further to the observation side than the substrate located at the rear as seen from the observation side of the 2 substrates.
By positioning the location of the said reflecting layer to the immediate rear of the liquid crystal layer, that is to say on the observer""s side of the liquid crystal substrate that is positioned to the very rear as seen from the observer""s side (at the front), ghosting does not occur on the display and a satisfactory display can be obtained, as parallax caused by the said substrate positioned at the very rear does not occur.
By using these means, the coloring of reflected light can be significantly improved compared with the past. However, to achieve a more achromatic coloring, corrections by adjusting the wavelength dispersion of the transmittance of parts such as color filters, polarizing plates and phase difference plates can be performed.
When correcting using color filters and phase difference plates, as it is necessary to suppress color device transmittance to the same coloring as that of the said light diffusion layer, the overall transmittance is lowered by the correction.
Therefore, the said liquid crystal display device of the present invention has, in addition, the special characteristic that when more than 1 polarizer is used and the refractive indices of the refractive index media of the light diffusion layer are
|nA(400)xe2x88x92nB(400)|xe2x89xa7|nA(700)xe2x88x92nB(700)|,
the chromaticity when 2 polarizers are parallel-arranged is a* less than 0, b*xe2x89xa60, and when
|nA(400)xe2x88x92nB(400)|xe2x89xa6|nA(700)xe2x88x92nB(700)|,
the chromaticity of the said 2 parallel-arranged polarizers is a* greater than 0, b*xe2x89xa70.
By this construction further improvements have been made and achromatic coloring of the display enhanced.
This is because the transmittance of the color device corresponding to the complementary color of the coloring of the aforesaid light diffusion layer has been heightened by weakening the degree of polarization.
Moreover, chromaticity a*, b* denotes the L*a*b* (CIELUV color space) coordinates of a uniform color space denoting chrominance perceived as of equivalent size, including cases where luminance differs, by expanding the UCS chromaticity diagram that displays chrominance to three dimensions. The said chromaticity range represents a coordinate domain of a coordinate plane with a* as the horizontal axis and b* as the vertical axis.
Furthermore, the present invention can be formed of a light dispersion layer and a high molecular liquid crystal. This is particularly advantageous when producing a diffraction grating layer. In this case, the refractive index anisotropy of 1 type of a high molecular liquid crystal is used to form what appear to be 2 types of refractive index medium. For example, domains are formed of a plurality of stripes or the like in the surface direction, and by changing the array direction of the liquid crystal molecules for each of the neighbouring domains, refractive index differences are imparted between the stripes. However, in this case, as the refractive index difference has polarity, it is necessary to select so that the difference becomes larger in the direction in which the light is particularly wanted to be diffused.